In the operation of bending windows by means of a press, it can, for various reasons, be important to heat the surfaces of the bending tools which come into contact with the glass. In this way it is possible to prevent the temperature of the glass from being lowered by contact with the bending tools. This is especially important when the windows are to be thermally tempered after the bending operation. Moreover, as in the case with windows which will not be tempered after the bending operation, the surface cooling of the glass produced by contact with the parts of the press can produce deformations in the curved window. Accordingly, it is also desirable to heat the bending molds to avoid this disadvantage.
Presently, there are various processes and devices for heating the bending tools to a temperature approximating the bending temperature of the glass. For example, bending molds have been constructed as solid molds with continuous bending surfaces and the heating devices have been disposed inside these molds (German A.S. 1,796,123). Devices of this type inevitably lead to deformation of the bending surfaces of the molds which is very difficult to control and correct.
In another process, the bending molds again consist of solid molds and the sheets of glass disposed between the bending tools are heated by means of gas burners (German A.S. 1,285,110, U.S. Pat. No. 3,333,935). In this instance, the gas burners not only heat the bending tools but also the sheets of glass. As a result, this process does not avoid the disadvantages arising from the contact of a hot sheet of glass with the substantially cooler bending tools. Moreover, the burners are not disposed in such a way to uniformly heat the entire surface of the sheet of glass and the bending tools.
There is also a process for bending sheets of glass in which flexible metal plates are placed between the hot sheets of glass and the faces of the bending tools. These metal plates are connected to a high frequency alternating current and the sheet of glass is heated by dielectric pulses (French Pat. No. 943,974). Apart from the expenditure required for high frequency generation and the necessary safety measures in this process, the sheet of hot glass is again placed in contact with cooler surfaces of the bending molds.